Supercontinuum light source comprising tapered microstructured optical fiber

ABSTRACT

The invention relates to a supercontinuum light source comprising a microstructured optical fiber and a pump light source. The microstructured optical fiber comprises a core and a cladding region surrounding the core, as well as a first fiber length section, a second fiber length section and an intermediate fiber length section between said first and second fiber length sections. The first fiber length section comprises a core with a first characteristic core diameter. The second fiber length section comprises a core with a second characteristic core diameter, smaller than said first characteristic core diameter, where said second characteristic core diameter is substantially constant along said second fiber length section. The intermediate length section of the optical fiber comprises a core which is tapered from said first characteristic core diameter to said second characteristic core diameter over a tapered length.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Ser. No. 15/942,908,filed on 2 Apr. 2018, which is a continuation of U.S. Ser. No.15/362,531, filed on 28 Nov. 2016, which is a continuation of U.S. Ser.No. 14/903,772, which was filed on 8 Jan. 2016, and which is a nationalstage application of PCT/DK2014/050205, which was filed on 7 Jul. 2014,and which claims the benefit of Danish Pat. App. No. PA 2013 70389,which was filed 10 Jul. 2013. The foregoing applications are herebyincorporated by reference.

TECHNICAL FIELD

The invention relates to a microstructured optical fiber suitable forgenerating supercontinuum light and the production of such an opticalfiber using for instance a drawing tower. Tapering of an optical fiberis advantageous in several systems, such as in a system forsupercontinuum generation, where the tapering of e.g. a non-linear fiberaccording to an embodiment of the present invention results insignificant improvements over the prior art systems.

BACKGROUND OF THE INVENTION

In optical fibers, a supercontinuum light is formed when a collection ofnonlinear processes act together upon feeding of a pump beam in order tocause spectral broadening of the original pump beam. The result may be asmooth spectral continuum spreading such as over more than an octave ofwavelengths. Suitable non-linear processes are for example self-phaseand cross-phase modulation, four-wave mixing, Raman gain or solitonbased dynamics, interacting together to generate the supercontinuumlight. In order to get the broadest continua in an optical fiber, it ismost efficient to pump in the anomalous dispersion regime; however aspectral continuum may in some situations be obtained by pumping in thenormal dispersion regime. Microstructured optical fibers are suitablefor supercontinuum generation due to their high non-linearity and theircustomizable zero dispersion wavelength.

The term “microstructured fibers” in this context is meant to coverfibers comprising microstructures such as photonic crystal fibers,photonic bandgap fibers, leaky channel fibers, holey fibers, etc. Unlessotherwise noted, the refractive index refers to the average refractiveindex which is usually calculated separately for the core and eachcladding layer surrounding it, whether the fiber is a standard fiber,where the core and any cladding layers surrounding that core have asubstantially homogeneous refractive index, or a microstructured fiberwhere the core and/or one or more cladding layers comprisemicrostructures. A cladding layer is defined a layer with a thicknessand surrounding the core where the refractive index is substantiallyhomogeneous or where the where the layer has a base material withsubstantially homogeneous refractive index and a plurality ofmicrostructures arranged in a uniform pattern.

A problem in relation to the present optical fibers for supercontinuumgeneration is that high optical peak power and/or pulse energies overtime damage(s) the optical fibers. The article “Analysis of thescalability of single-mode near-infrared supercontinuum to high averagepower” by Rui Song et al, published 29 Jan. 2013 in IOP Publishing,Journal of Optics, J. Opt. 15 (2013) 035203 examines the restrictionsimposed by thermal and nonlinear effects, fiber end facet damage, pumpand fiber combiner limits, and damage to the amplifier fibers. In termsof nonlinear effects, the restriction is the self-focus effect, whosethreshold is around 4 MW. The fiber end facet damage threshold limitsthe power density to 10 μm⁻², but the damage threshold can be improvedby using an end-cap with a large diameter. Here the end-cap consists ofpure fused silica. By expanding the fiber mode in the end-cap, the powerdensity on the surface can be reduced to under the average output power,so that the surface damage threshold is increased.

Supercontinuum generation is a complex process, and any quantitativeexplanation of the underlying physics must take into account a number ofdifferent fiber and pulse parameters. Nonetheless, it is generallyaccepted that the most efficient method to obtain a very broadsupercontinuum is by using a pump wavelength slightly in the anomalousgroup-velocity dispersion (GVD) regime of a highly nonlinear PhotonicCrystal Fiber (PCF) with only one zero-dispersion wavelength (ZDW) belowthe absorption limit of the material. In contrast pumping in the normalGVD regime of a PCF will in general reduce the bandwidth and require alonger length of the PCF (J. Dudley et al, “Supercontinuum generation inphotonic crystal fiber”, Reviews of Modern Physics, Vol. 78, p. 1135,October-December 2006).

Normally high power supercontinuum sources use a pump wavelength ofaround 1064 nm and a PCF with a core size of about 3.5 to 5 μm. Astandard calculation of the dispersion shows that the core size of a PCFhaving ZDW of 1064 nm increases with the hole size, and for very largehole sizes it reaches about 6 μm. Hence in order to have anomalousdispersion at 1064 nm in a PCF the core size is limited to around 6 μmor less.

E.g. K. K. Chen report a “Picosecond fiber MOPA pumped supercontinuumsource with 39 W output power” (Optics Express, Vol. 18, No. 6, p. 5431,15 Mar. 2010). They used a 21 ps 1060 nm laser which is pumped into a 2m long PCF with a core size of 4.4 μm and a ZDW of 1012 nm and foundthat the maximum average power was limited by thermal damage. Hence theyconcluded that power scaling might therefore be possible by using amode-expanding end-cap at the input to the fiber.

G. Genty et al report normal pumped supercontinuum in PCFs with cores upto 20 μm in the paper “Supercontinuum generation in large mode-areamicrostructured fibers” (Optics Express, Vol. 13, No. 21, p. 8625, 17Oct. 2005). Here it is shown that a supercontinuum spanning more than anoctave can be generated by a 3 ns 1064 nm pump in a 100 m long fiberwith a 10 μm core, and the mechanisms leading to the continuum in thiscase primarily rely on the processes of cascaded stimulated Ramanscattering and four-wave mixing. It is concluded that the large area ofthe fibers should allow for the generation of extremely high powersupercontinuum as the damage threshold is considerably increased.However, it is observed that the normalized intensity is significantlyhigher above than below the pump wavelength of 1060 nm, and that afurther increase of the core size leads to a significant decrease of thepower on the blue side of the spectrum.

C. Xiong et al have reported another example of normal pumpedsupercontinuum in “Enhanced visible continuum generation from amicrochip 1064 nm laser” (Optics Express, Vol. 14, No. 13, p. 6188, 26Jun. 2006). Here a tapered fiber approach was used, the first fibersection has a core size of 5 μm and a ZDW of 1103 nm. The proximity ofthe ZDW to pump wavelength allows for a strong four-wave mixing gain to742 nm, allowing >35% conversion of the 1064 nm pump light over a 3 mlong fibre. The fiber is tapered to a core size of 1.7 μm giving a ZDWof 700 nm, as previous studies have shown that small-core PCF are idealfor super continuum generation from pulsed sources at wavelengths from600 to 800. This fiber gives a bright single mode visible light sourcewith output power of up to 20 dB/m.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical fiber forgenerating supercontinuum light, with a relatively low risk of fiberdamage even where the fiber is arranged to provide high pulse energy.

In an embodiment it is an object to provide an optical fiber forgenerating supercontinuum light with an increased output power comparedwith present optical fibers.

It is another object of the invention to provide a supercontinuum lightsource that is able to output supercontinuum light with high outputpower while simultaneously having a relatively low risk of fiber damage.

In an embodiment it is an object to provide a supercontinuum lightsource with an increased output power compared to present supercontinuumlight sources.

The supercontinuum light source of the invention is advantageouslysuitable for use in an optical measurement system.

In an embodiment of the present invention a microstructured opticalfiber for generating supercontinuum light upon feeding of light having afirst wavelength λ₁ is provided. The optical fiber has a length and alongitudinal axis along its length and comprises a core region forguiding light along the length of the optical fiber, and a firstcladding region surrounding the core region. Along its length, theoptical fiber comprises a first fiber length section, a second fiberlength section as well as an intermediate fiber length section betweenthe first and second fiber length sections. The first fiber lengthsection has a core region with a first characteristic core diameter W₁in a cross-section through the fiber perpendicularly to the longitudinalaxis, wherein the first characteristic core diameter is larger thanabout 7 μm. The second fiber length section has a core region with asecond characteristic core diameter W₂ in a cross-section through thefiber perpendicularly to the longitudinal axis, where the secondcharacteristic core diameter W₂ is smaller than the first characteristiccore diameter W₁. The intermediate fiber length section of the opticalfiber comprises a core region which is tapered from the firstcharacteristic core diameter W₁ to the second characteristic corediameter W₂ over a tapered length L_(i). The first fiber length sectionhas normal dispersion at the first wavelength λ₁ and the second fiberlength section has zero dispersion at a second wavelength ZDW₂, whereZDW₂ is up to about λ₁+50 nm. The second fiber length section hasanomalous dispersion at wavelengths above the second wavelength ZDW₂.

The microstructured optical fiber has shown to have a very highresistance against fiber damage even where the fiber is arranged toprovide high pulse energy, and it has been found that a very broad highpower supercontinuum may be generated. Due to the relatively large firstcharacteristic core diameter of the first fiber length section, largeamount of light may be fed to the fiber while the relatively smallersecond characteristic core diameter of the second fiber length sectionensures that a very broad supercontinuum may be generated even with highpower on the blue side of the spectrum.

In order to provide a desired broad supercontinuum it is preferred thatthe first wavelength λ₁ is about 1100 nm or less. More preferably thefirst wavelength λ₁ is from about 700 nm to about 1100 nm, such as fromabout 900 nm to about 1100 nm.

The characteristic core diameter is a term applied in the context of thepresent specification as a generalization that enables determination ofthe diameter of the core even if the core of the fiber is not circular.Another definition of the characteristic diameter of the core of a fiberis the outer diameter of a refractive index profile which providesguiding when light is launched in the center of the fiber (assumingcylinder symmetry).

The core is normally located along the centre axis of the fiber and isusually surrounded by one or more cladding regions. Other shapes ofcores are also possible, such as elliptical cores, and a fiber maycomprise more than one core. The cladding region(s) is/are often furthersurrounded by one or more coating(s) and/or other layer(s) oftensuitable for providing environmental and/or mechanical shielding.

Light is normally guided in the core by refraction and/or total internalreflection due to a higher refractive index in the core relative to thecladding. The average refractive index of the core and/or cladding(s)may be engineered by doping the base material and/or by introducingmicrostructures running along the length of the fiber.

In a microstructured optical fiber, the core area is usually defined asthe area of a circle inscribed by the elements of the fiber arranged toimmediately surround the core. In the context of the present invention,the phrase “characteristic core diameter” of a microstructured opticalfiber refers to the diameter of the largest circle that may be inscribedwithin the core without interfering with any such elements of the fiber,in a cross-section through the fiber perpendicularly to the longitudinalaxis thereof.

The microstructured optical fiber or simply called the optical fibercomprises at least one tapered length and is therefore also referred toas a tapered microstructured optical fiber. However, it should beunderstood that the phrase “tapered microstructured optical fiber” doesnot mean that the microstructured optical fiber is tapered along itswhole length but merely that it comprises at least one tapered lengthL₁.

The phrase “normal dispersion” is well-known to the skilled person anddenotes that the dispersion D satisfies: D<0 ps/nm/km. The term “zerodispersion wavelength” denotes a wavelength where the group delaydispersion, i.e. the second-order dispersion, of the light is zero. Itis also well-known to the skilled person that when the dispersion isanomalous, the rate of change of the index of refraction with respect tothe wavelength changes sign compared to the normal dispersion range, andthe group velocity of light is no longer an indicator of signalvelocity.

The first fiber length section of the microstructured optical fiber ispreferably arranged to facilitate a substantial unchanged spectraldistribution of light passing through the core of the first fiber lengthsection of the optical fiber.

In an embodiment the microstructured optical fiber is fed with light atone or more further wavelengths, such as at wavelengths close to halfthe first wavelength λ₁.

The intermediate fiber length section of the microstructured opticalfiber has a core region with a non-constant characteristic core diameterat least along part of the intermediate fiber length section, and themicrostructure of the optical fiber exists at least along a part of thelength of the intermediate fiber length section of the optical fiber.

The phrases “first fiber length section” and “second fiber lengthsection” are not intended to limit the scope of the claims to situationswhere light propagates from the first fiber length section towards thesecond longitudinal position. The tapered microstructured optical fiberalso relates to embodiments, wherein light propagates from the secondfiber length section towards the first fiber length section.

In an embodiment the tapered microstructured optical fiber is configuredfor propagating light from the first fiber length section towards thesecond fiber length section.

In the context of the present invention, the phrase “supercontinuum”refers to a spectrally broad signal. The supercontinuum is said to havea “blue edge” defining the lower boundary of the spectrum and a “rededge” defining the upper boundary of the spectrum. In a silica opticalfiber, the blue edge may advantageously be at a wavelength in the rangeof 300 nm to 600 nm, and the red edge may advantageously be at awavelength in the range of 1300 nm to 2400 nm, such as in the range of1600 nm to 2400 nm. The spectral width of the supercontinuum is definedas the difference between the wavelength of the red and blue edges.

In an embodiment, the second fiber length section of the optical fiberhas a zero dispersion wavelength in the range from about λ₁−150 nm toabout λ₁+50 nm. Fibers having such zero dispersion wavelengthsrelatively close to the pumping wavelength have been found to be verywell suited for broad supercontinuum generation.

In an embodiment, the tapered length L_(i) is at least about 0.5 m, suchas larger than about 1 m, such as larger than about 1.5 m, such aslarger than about 2 m, such as larger than about 3 m, such as largerthan about 5 m. Thereby, the tapering is advantageously carried out in adrawing tower during the generation of the optical fiber as opposed toin a tapering station. By having a relatively long tapered length L_(i)it has been found that even higher output power is obtainable withoutresulting in damage of the microstructured optical fiber.

In an embodiment, the first fiber length section has zero dispersion ata wavelength ZDW₁ where ZDW₁> about λ₁+20 nm, preferably ZDW₁> aboutλ₁+40 nm, preferably ZDW₁> about λ₁+60 nm, preferably ZDW₁> about λ₁+80nm, preferably ZDW₁> about λ₁+100 nm. Hereby, the first fiber lengthsection of the microstructured optical fiber is arranged to facilitate asubstantially unchanged spectral distribution of light passing throughit. Thereby the supercontinuum generation will take place mainly in thesecond fiber length section and optionally partly in the intermediatefiber length section.

In an embodiment the first characteristic core diameter is larger thanabout 1.5 times the second characteristic core diameter, such as largerthan about 2 times the second characteristic core diameter, such aslarger than about 3 times the second characteristic core diameter, suchas larger than about 4 times the second characteristic core diameter,such as larger than about 5 times the second characteristic corediameter.

In an embodiment the first characteristic core diameter is larger thanabout 8 μm, such as larger than about 10 μm, such as larger than about15 μm, such as larger than about 20 μm, such as larger than about 25 μm.

In an embodiment the second characteristic core diameter is larger thanabout 3 μm, such as larger than about 3.5 μm, such as larger than about4 μm, such as larger than about 4.5 μm, such as larger than about 5 μm,such as larger than about 5.5 μm.

The invention provides a new concept where the first fiber lengthsection may be designed for optimal light feeding at a high pulse energyand the second fiber length section may be designed for optimalsupercontinuum generation and further the intermediate fiber lengthsection may designed for optimal connecting of the first and the secondfiber length section e.g. to provide substantially adiabatic transfer oflight along the core of the intermediate fiber length section. Based onthe teaching of the present invention the skilled person will be able tooptimize the individual sections to thereby obtain a microstructuredfiber of the invention with high damage threshold and with optimalproperties for a specific application.

In an embodiment the inner cladding comprises a microstructure, wherethe microstructure comprises a plurality of microstructure elementshaving a microstructure element diameter, d_(f). The microstructureelements are arranged at a pitch ∧, where the pitch ∧ is a measure of aspacing between the microstructure elements. Advantageously themicrostructure is at least partially maintained along both the first andsecond fiber length sections of the optical fiber. The microstructureelement diameter, d_(f), the pitch ∧ of the microstructure elements aswell as the characteristic diameter W of the core are illustrated inFIGS. 2 and 3 and will be described in more details below. In thecontext of the present invention, the phrase “microstructure ispartially maintained” means that the microstructure is present in someof or all of the first fiber length section as well as being present insome of or all of the second fiber length section. As an example, themicrostructure might be collapsed at an end of the first and/or secondfiber length section, providing an end cap, whilst it is presentthroughout the rest of the first and second fiber length sections.

In an embodiment, the relative size (d_(f)/∧) of the microstructureelements is substantially equal in the first and second fiber lengthsections of the optical fiber, where the relative size is the ratiobetween the diameter of the microstructure elements d_(f) and the pitch∧. As an example, the microstructure elements are holes or elementshaving a lower refractive index than the base material of the opticalfiber.

The term “substantially” as used herein means to include what is withinthe ordinary tolerances.

In an embodiment the relative size d_(f)/∧ of the microstructureelements is larger in the second fiber length section of the opticalfiber than in the first fiber length section of the optical fiber, therelative size being the ratio between the diameter d_(f) of themicrostructure elements and the pitch ∧. In this embodiment, therelative size of the microstructure elements preferably variesthroughout at least a part of the intermediate fiber length section ofthe optical fiber. Thus, the size of the microstructure elements is forexample changed as the core size is changed. It is advantageous that therelative size d_(f)/∧ of the microstructure elements is larger in thesecond fiber length section than in the first fiber length section, inthat relative large d_(f)/∧ may assists in avoiding multimode light atlarge core sizes of the first fiber length section. In an embodiment,the change in relative size d_(f)/∧ of the microstructure elements isprovided by pressure control during drawing of the optical fiber in adrawing tower. A control of the relative size d_(f)/∧ of themicrostructure elements is for example as described in patentapplication EP1153325.

In an embodiment the relative size of the microstructure elements ischosen so that the first fiber length section is a single mode fiber atleast at the first wavelength λ₁.

According to an embodiment, the optical fiber comprises an end capadjacent to the first fiber length section. The end cap isadvantageously adjacent to a first end of the first fiber length sectionopposite a second end of the first fiber length section adjacent to theintermediate fiber length section of the microstructured optical fiber.The term “end cap” is meant to denote a fiber end part withoutmicrostructures and providing an input to or an output from the opticalfiber. Thus, the term “end cap” covers both a piece of glass materialadded to a microstructured fiber as well as an end part of themicrostructured optical fiber itself, where no microstructures exist dueto e.g. collapsing of the microstructures.

The end cap may advantageously have a uniform refractive index. In anembodiment the end cap has a length of up to 100 times the firstcharacteristic core diameter.

In an embodiment, the optical fiber comprises an end cap at the inputend of the microstructured optical fiber thereby providing an evenhigher damage threshold and simultaneously providing a fiber endsuitable for splicing.

In an embodiment, the optical fiber comprises an end cap at the outputend of the microstructured optical fiber e.g. to provide a fiber endsuitable for splicing.

In an embodiment, the microstructured optical fiber comprises a thirdfiber length section with a third characteristic core diameter W₃ and asecond intermediate fiber length section between the second fiber lengthsection and the third fiber length section, where the secondintermediate fiber length section comprises a core region which istapered from the second characteristic core diameter W₂ to the thirdcharacteristic core diameter W₃, where W₃ is larger than W₂ In oneembodiment, W₃ is substantially equal to W₁.

In an embodiment, the optical fiber further comprises amulti-cladding-structure provided by a second cladding surrounding thefirst cladding, where the core region is adapted to guide an opticalsignal at the first wavelength λ₁. The core region has an effectiverefractive index n_(core), and the core region comprises a materialdoped with at least one rare earth element. The first cladding isarranged for guiding light at a third wavelength λ₃. The first claddingpreferable has an effective refractive index n_(first-clad), and thesecond cladding has an effective refractive index n_(second-clad),wherein n_(core)>n_(first-clad)>n_(second-clad) and λ₁>λ₃. In anembodiment the multi-cladding structure features of the optical fiberare present in the first, second and intermediate fiber length sectionof the microstructured optical fiber. In an embodiment where the opticalfiber comprises a second intermediate fiber length section and a thirdfiber length section, the multi-cladding structure is also present inthese fiber length sections. Thus, in this embodiment the first, secondand intermediate fiber length sections of the microstructured opticalfiber, as well as the second intermediate fiber length section and thethird fiber length section, if present in the optical fiber, all containrare earth elements. The core region of this optical fiber in thisembodiment is thus a cladding pumped amplifier waveguide for amplifyingan optical signal at a wavelength λ₁. In this embodiment, the firstcladding is an inner cladding, whilst the second cladding is an outercladding surrounding the first cladding, wherein the inner cladding isadapted to guiding pump light whilst the outer cladding is adapted tokeeping pump light inside the outer cladding. In an embodiment, λ₃ iswithin the absorption band of the rare earth element, e.g. theabsorption band of Ytterbium. By this embodiment an even higher powersupercontinuum may be generated.

In an embodiment, the microstructuring of the microstructured opticalfiber comprises microstructure elements of a microstructure materialembedded in a base material, wherein the refractive index of the core,n_(core), is equal to the refractive index of the base material. In anembodiment, the refractive index of the core, n_(core) is substantiallyequal to the refractive index of silica glass.

In an embodiment, the at least one rare earth element is Ytterbium (Yb)and/or Neodymium (Nd).

As mentioned above, the invention also relates to a supercontinuum lightsource comprising a microstructured optical fiber for generatingsupercontinuum light at a wavelength λ. The microstructured opticalfiber comprises a core region that is capable of guiding light along alongitudinal axis of the optical fiber, and a first cladding regionsurrounding the core region. The microstructured optical fiber furthercomprises a pump light source arranged to feed light into the coreregion at an input end of the optical fiber, where the light has a firstwavelength, λ₁. The optical fiber has a length and a longitudinal axisalong its length, wherein the optical fiber comprises a first fiberlength section, a second fiber length section as well as an intermediatefiber length section between the first and second fiber length sections.The first fiber length section has a core region with a firstcharacteristic core diameter W₁, wherein the first characteristic corediameter is larger than about 7 μm; the second fiber length section hasa core region with a second characteristic core diameter W₂ in across-section through the microstructured optical fiber perpendicularlyto the longitudinal axis, where the second characteristic core diameterW₂ is smaller than the first characteristic core diameter W₁. Theintermediate fiber length section of the optical fiber comprises a coreregion which is tapered from the first characteristic core diameter W₁to the second characteristic core diameter W₂ over a tapered lengthL_(i).

The first fiber length section preferably has normal dispersion at afirst wavelength λ₁ up to about 1100 nm, more preferably from about 900nm to about 1100 nm and the second fiber length section has zerodispersion at a second wavelength ZDW₂, where ZDW₂ is smaller than up toabout λ₁+50 nm, and the second fiber length section has anomalousdispersion at wavelengths above the second wavelength ZDW₂.

Advantageously the optical fiber of the SC light source is as describedin one or more of the above embodiments.

In an embodiment the pulse duration Δt of the pump light source is morethan about 1 ps, such as more than about 5 ps, such as more than about10 ps, such as more than about 50 ps, such as more than about 100 ps,such as more than about 500 ps, such as more than about 1 ns, such asmore than about 2 ns, such as more than about 5 ns, such as more thanabout 10 ns, such as more than about 50 ns, such as more than about 1ms. The output pulse length is controllable by controlling the pumpcharacteristics, i.a. the pump pulse length.

In an embodiment the pulse duration Δt of the pump light source is lessthan about 1 ms, such as less than about 50 ns, such as less than about10 ns, such as less than about 5 ns, such as less than about 2 ns, suchas less than about 1 ns, such as less than about 500 ps, such as lessthan about 100 ps, such as less than about 50 ps, such as less thanabout 10 ps, such as less than about 5 ps, such as less than about 1 ps.

In an embodiment the pulse duration Δt of the pump light source is morethan about 1 ps, such as more than about 5 ps, such as more than about50 ps such as more than about 100 ps, such as more than about 500 ps,such as more than about 1 ns, such as more than about 2 ns, such as morethan about 5 ns. Relative long pulses are in particular suitable for usein photoacoustic applications where high energy pump pulses normally aredesirable.

It is well-known to the skilled person that the output pulse length of asupercontinuum light source is controllable by controlling thecharacteristics of the light fed into the optical fiber, i.e. bycontrolling the pump pulse characteristics.

In an embodiment, the output pulse from the supercontinuum light sourcehas a pulse length of less than about 20 ns and an output spectraldensity over at least 50 nm between 400 nm and 900 nm such as at least100 nJ/10 nm, such as at least 200 nJ/10 nm, such as at least 300 nJ/10nm, such as at least 500 nJ/10 nm, such as at least 1000 nJ/10 nm.Hereby, the supercontinuum light source is suitable for photoacousticapplications.

In an embodiment the pump light source comprises a mode-locked laser andat least one amplifier, and the supercontinuum light source has anoutput spliced onto the input end of the optical fiber. In thisembodiment, the connection feeding light from the pump light source tothe optical fiber does not comprise any free space couplings. In thisembodiment, the number of splicings and/or intermediate fibers isreduced, which makes it possible to produce the optical fiber fastercompared to an optical fiber with more splicings. Further, any risk ofpower loss due to splicings is reduced due to a reduced number ofsplicings. This embodiment is in particular suitable for systems withpicosecond pulses having one or more amplifier.

In an embodiment, the supercontinuum light source is used for at leastone of the following applications: photoacoustic measurements,multi-spectral imaging, LIDAR (Light Detection and Ranging), STED(Stimulated Emission Depletion).

The inventors further disclose a pump light source comprising a systemfor active feedback.

The pump light source may advantageously be used as a part of thesupercontinuum light source of the invention but it may in principle beapplied as a light source in any optical systems such as in asupercontinuum light source based on any SC generating optical fiber,such as any prior art non-linear optical fiber suitable forsupercontinuum generation.

The pump light source comprises a laser diode arranged to feed light toa pump laser such as a q-switched laser. The pump light source furthercomprises a photodiode and a control unit. The photodiode is arranged toreceive part of an emitted pulse, preferably incident light, from thepump diode. The light received is preferably a minor amount of theemitted pulse, such that the power of the pump laser is notsubstantially reduced thereby. The photodiode is arranged to transmit asignal to the control unit upon receipt of part of the emitted pulsesfrom the pump laser and the control unit is arranged to shut of thelaser diode e.g. via transmission of a signal upon receipt of the signalfrom the pump laser.

The signal from the photodiode is sent to a control unit 33, which sendsa signal to the laser diode to shut it “OFF” thus preventing furtherlight to be pumped into the laser cavity.

The pump light source with the system for active feedback adds toimproved control of the emission frequency (repetition rate) of the pumplaser, and/or to reduce the timing jitter between the pump laser pulses.

In one embodiment the control is obtained by modulating the output fromthe laser diode in dependence on the photodiodes detection of whenever apulse is emitted from the pump laser. The feedback signal is used toswitch the laser diode from “ON” (emitting light) to “OFF” (noemission).

The photodiode should advantageously be positioned such that it receivespart of the emitted pulses from the pump laser (e.g. 064 nm), but suchthat it is substantially not sensitive to light emitted from the laserdiode (e.g. 808 nm) as light from the laser diode may give rise to noisein the detection. This may be obtained by either careful selection ofphotodiode type or by placing an appropriate filter in front of thedetector.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained in more detail below in connection withpreferred embodiments and with reference to the drawings in which:

FIG. 1 is a schematic drawing of a cross-section of a microstructuredfiber, along the longitudinal axis;

FIG. 2 shows a picture of a cross-section of a microstructured fiber,perpendicular to the longitudinal axis;

FIG. 3 shows part of a cross-section of a microstructured fiber,perpendicular to the longitudinal axis;

FIG. 4a shows a schematic drawing of a microstructured optical fiberaccording to the invention;

FIGS. 4b and 4c show cross-sections of a microstructured optical fiber,perpendicular to the longitudinal axis, at a first and second fiberlength section, respectively;

FIG. 5 is a cross-section of a microstructured optical fiber accordingto the invention, along to the longitudinal axis;

FIG. 6 is a schematic drawing of a supercontinuum light sourcecomprising a microstructured optical fiber and a pump light source;

FIG. 7 is a graph showing dispersion curves for two optical fibers as afunction of wavelength;

FIG. 8 is a supercontinuum spectrum from a supercontinuum light sourceof the invention; and

FIG. 9 is an illustration of a pump light source suitable for asupercontinuum light source and comprising a system for active feedback.

The figures are schematic and are simplified for clarity. Throughout,the same reference numerals are used for identical or correspondingparts.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

The invention is defined by the features of the independent claim(s).Preferred embodiments are defined in the dependent claims. Any referencenumerals in the claims are intended to be non-limiting for their scope.

Some preferred embodiments have been described in the foregoing, but itshould be stressed that the invention is not limited to these, but maybe embodied in other ways within the subject-matter defined in thefollowing claims.

With reference to FIG. 1 is described a schematic drawing of across-section of a microstructured fiber, along the longitudinal axis.The cross-section is an enlarged view of a very short length ofmicrostructured fiber 1. The microstructured fiber 1 has a core region 2and a cladding region 3 surrounding the core region 2.

The microstructured optical fiber 1 has a length consisting of a length7 of the microstructured fiber as well as a length 8 of an end cap, aswell as an outer diameter D. The microstructures of the microstructuredfiber 1 are holes. These holes have been collapsed as shown in thefigure in the area of fiber length 7 adjacent to the end cap 8. One wayof carrying out such a collapse is by heating of the optical fiber 1.

In this prior art embodiment, the microstructured fiber is a non-taperedfiber. The characteristic diameter of the core is of the order ofmagnitude “some μm”, typically 3.5 or 5 μm; the extension of the end caplength 8 in the longitudinal axis of the fiber is of a magnitude of 100μm, e.g. 200 μm, whilst the microstructured fiber length is e.g. severalmeters, for example 10 meters.

Light has been fed into the fiber; the reference number 4 indicateslight confined within the core region of the fiber 1, reference number 5indicates light spreading from the core region of the length 7 of thefiber into the end cap 8, whilst reference number 6 indicates lightexiting from the end cap 8 of the optical fiber 1. It may be seen thatthe light spreads out in the region of the collapsed microstructures andin the end cap 8 compared to the length 7.

FIG. 2 shows a picture of a cross-section of a microstructured fiber 1,perpendicular to the longitudinal axis. The fiber is a microstructuredfiber comprising a core region 2 and a cladding region 3, the claddingregion surrounding the core region. The cladding region compriseslow-index cladding features 9, here including features in the form ofholes or voids extending in the longitudinal direction of the fiber, andan inner cladding background or base material. The core region 2comprises a refractive index profile such that the core region comprisesmaterial with a refractive index n_(core) being different from therefractive index of a material in the inner cladding region. In order totune various properties of the optical fiber it may be preferred to havea special refractive index profile of the core region. The region Adenotes an area of the fiber to be shown enlarged in FIG. 3.

FIG. 3 shows part of a cross-section of a microstructured fiber,perpendicular to the longitudinal axis, corresponding to an enlargementof the square region denoted A in FIG. 2. In FIG. 3 is shown the corearea or core region 2 as defined as the area of a circle inscribed bythe microstructure elements of the cladding arranged to immediatelysurround the core 2. The circle has characteristic core diameter W beingthe diameter of the largest circle that may be inscribed within the corewithout interfering with any microstructure elements or claddingfeatures of the fiber, in a cross-section through the fiberperpendicularly to the longitudinal axis thereof. The cladding comprisesa microstructure with a plurality of microstructure elements or claddingfeatures each having a microstructure element diameter d_(f), and themicrostructure elements are arranged at a pitch ∧, where the pitch ∧ isa measure of the spacing between the microstructure elements. As shownin FIG. 3 the pitch ∧ is indicated as the distance between the centersof two adjacent microstructure elements.

FIG. 4a shows a schematic drawing of a microstructured optical fiber 10according to the invention, and FIGS. 4b and 4c show cross-sections ofthe microstructured fiber, perpendicular to the longitudinal axis, at afirst and second fiber length section, respectively. The microstructuredoptical fiber is arranged for generating supercontinuum light uponfeeding of light having a first wavelength λ₁ preferably up to about1100 nm, such as from about 900 nm to about 1100 nm. The optical fiber10 has a length and a longitudinal axis along its length and comprises acore region for guiding light along the length of the optical fiber, anda first cladding region surrounding the core region.

The optical fiber 10, along its length, comprises a first fiber lengthsection 12, a second fiber length section 14 as well as an intermediatefiber length section 13 between the first and second fiber lengthsections 12, 14.

In one embodiment the total length of the optical fiber 10 is such asless than about 50 m, such as less than about 30 m, such as less thanabout 20 m.

FIG. 4b shows a cross-section of the microstructured fiber,perpendicular to the longitudinal axis, at the first length section 12.It is indicated that the first fiber length section 12 has a core regionwith a first characteristic core diameter W₁ in a cross-section throughthe microstructured optical fiber perpendicularly to the longitudinalaxis. The first characteristic core diameter W₁ is larger than about 7μm. The second fiber length section 14 has a core region with a secondcharacteristic core diameter W₂ in a cross-section through themicrostructured optical fiber perpendicularly to the longitudinal axis,where the second characteristic core diameter W₂ is smaller than thefirst characteristic core diameter W₁. The microstructure elements ofthe microstructured optical fiber at the first fiber length section 12are arranged at a pitch ∧₁ and have a microstructure element diameterd₁, whilst the microstructure elements of the microstructured opticalfiber at the second fiber length section 14 are arranged at a pitch ∧₂and have a microstructure element diameter d₂, as indicated in FIGS. 4band 4 c.

The intermediate fiber length section 13 of the optical fiber 10comprises a core region which is tapered from the first characteristiccore diameter W₁ to the second characteristic core diameter W₂ over atapered length L_(i).

The first fiber length section 12 has normal dispersion at the firstwavelength λ₁ and the second fiber length section has zero dispersion ata second wavelength ZDW₂, where ZDW₂ is up to about λ₁+50 nm. The secondfiber length section has anomalous dispersion at wavelengths above thesecond wavelength ZDW₂.

FIG. 5 shows a cross-section of the tapered microstructured opticalfiber 10 along the longitudinal axis. From FIG. 5 it may be seen thatthe structure of the microstructured optical fiber 10 comprising a coreregion 2 and a cladding region 3 is maintained throughout the firstfiber length section 12, the tapering in the intermediate fiber lengthsection 13 and the second fiber length section 14. As explained inrelation to FIGS. 4a-4c , the size of the core region as well as thepitch and the microstructure diameter differ in the different fiberlength sections 12, 13, 14 of the microstructured optical fiber, but thenumber of microstructures is kept, except at the end cap region 8 of themicrostructured optical fiber 10. The vertical hatched lines in FIG. 6indicates the transition between the first fiber length section 12 andthe intermediate fiber length section 13 and between the second fiberlength section 14 and the intermediate fiber length section 13.

Referring again to FIGS. 4a-4c , it may be seen from FIG. 5a that thetapering from the first fiber length section to the second fiber lengthsection means a substantially monotonic decrease of the dimensions ofthe microstructured optical fiber from the first length section 12 tothe second fiber length section 14. In the first fiber length section12, the first characteristic core diameter W₂ is substantially constant,and in the second fiber length section 14, the second characteristiccore diameter W₂ is substantially constant. Moreover, the microstructureelement pitch ∧₁ and microstructure element diameter d₁ aresubstantially constant in the first fiber length section 12; themicrostructure element pitch ∧₂ and microstructure element diameter d₂are substantially constant in the second fiber length section 14, whilstthe microstructure pitch and microstructure element diameter differalong at least a part of the intermediate fiber length section 13 of themicrostructured optical fiber 10.

FIG. 5 shows a cross-section through a microstructured tapered fiber 10according to an embodiment of the invention, along the longitudinal axisof the microstructured optical fiber. The microstructured optical fibercomprises an end cap 8, a first fiber length section 12, an intermediatefiber length section 13 and a second fiber length section 14. Thevertical dashed lines indicate the transition between the end cap 8 andthe first fiber length section, between the first fiber length section12 and the intermediate fiber length section 13 and the transitionbetween the intermediate fiber length section 13 and the second fiberlength section 14, respectively.

FIG. 5 shows that the microstructured optical fiber 10 has a core region2 and a cladding region 3. The cladding region comprises low-indexcladding features 9, for example features in the form of holes or voidsextending in the longitudinal direction of the microstructured opticalfiber, and an inner cladding background or base material in which thecladding features are embedded. FIG. 5 shows that the core region 2 andthus the characteristic core diameter is larger in the first fiberlength section 12 than in the second fiber length section 14, whilst thecharacteristic core diameter changes along the length of theintermediate fiber length section 13. Moreover, FIG. 5 shows that thenumber of low-index cladding features 9 is unchanged in the first fiberlength section 12, the intermediate fiber length section 13 and thesecond fiber length section 14. However, the cladding region 3 is largerin the first fiber length section 12 than in the second fiber lengthsection 14, whilst the size of the cladding region changes along thelength of the intermediate length section 13.

As an example only, the characteristic core diameter at the first fiberlength section is 5 μm and the characteristic core diameter at thesecond fiber length section is 10 μm.

It is an insight of the inventors that fiber damage due to high opticalpeak powers and/or pulse energies often takes place at the transitionbetween the end cap and the first fiber length section 12, and not atthe end cap input of the microstructured optical fiber. Therefore, byproviding a fiber with enlarged input dimensions in the form of a fiberwith a first fiber length section having enlarged core diameter comparedto the second fiber length section, it is achieved that themicrostructured optical fiber is able to receive more peak power and/orpulse energy without being damaged. It is believed that this is due tothe effect that the light fed into the first fiber length section isdistributed over the large core in the first fiber length section.

FIG. 6 is a schematic drawing of a supercontinuum light source 100comprising a microstructured optical fiber 10 and a pump light source20. The pump light source 20 has an output 25 arranged to feed lightinto the end cap 8 of the microstructured optical fiber 10, adjacent tothe first fiber length section 12 of the optical fiber. The light fedinto the end cap 8 of the optical fiber 10 continues to the intermediatefiber length section 13 and the second fiber length section 14. Due tothe large size of the core of the microstructured optical fiber in thefirst fiber length section 12, a large amount of light may be fed intothe microstructured optical fiber 10 without damaging it. The light isconfined to the core region, and as the core region of themicrostructured optical fiber is reduced throughout the intermediatefiber length section, the intensity of the confined light increases.However, due to the relatively long intermediate fiber length section13, the transition of the light intensity from the first fiber lengthsection 12 to the second fiber length section 14 takes placeadiabatically.

FIG. 7 is a graph showing dispersion curves for two optical commerciallyavailable fibers, LMA-10 and SC-5.0-1040 (NKT Photonics A/S), as afunction of wavelength λ. The dispersion curve for the optical fiberLMA-10 is shown by a solid curve, whilst the dispersion curve for theoptical fiber SC-5.0-1040 is shown by a dashed curve. FIG. 8 shows thatthe zero dispersion wavelength, ZDW_(LMA), for the optical fiber LMA-10is about 1180 nm, whilst the zero dispersion wavelength, ZDW_(SC), forthe optical fiber SC-5.0-1040 is about 1040 nm. As the skilled personknows, the area below the horizontal line indicating zero dispersioncorresponds to normal dispersion, whilst the area above the horizontalline indicating zero dispersion corresponds to anomalous dispersion.From this it can be seen that the first fiber length section in anembodiment of the microstructured fiber of the invention for example mayhave a cross-sectional structure corresponding to the optical fiberLMA-10, such that ZDW₁=ZDW_(LMA), and the second fiber length sectionmay have a cross-sectional structure corresponding to the optical fiberSC-5.0-1040, such that ZDW₂=ZDW_(SC).

FIG. 8 is a supercontinuum spectrum from a supercontinuum light source100 of the invention. The supercontinuum light source 100 comprises apump light source 20 and a microstructured optical fiber for generatingsupercontinuum (SC) light (microstructured SC fiber 10).

The pump laser 20 applied was a passive q-switched laser, comprising aNd:YAG crystal combined with a Cr:YAG. The backside of the Nd:YAG wascoated with HR coating at 1064 nm and a semi-transparent mirror with a60% reflection@1064 was placed in front of the Cr:YAG. The cavity waspumped using an 808 nm diode pump. The resulting laser cavity emittedpulsed light at 1064 nm with a repetition rate of app. 18-20 kHz and apulse width of 1.2 ns. Measured output power was 600 mW, but with aportion of this light arising from non-absorbed pump laser light(residual 808 nm light).

The pump laser was attempted coupled into the commercially availablefiber SC-5.0-1040 from NKT Photonics. The fiber has pitch ∧=3.3 μm, andrelative hole size d/∧=0.52 giving core size 5 μm. It was observed thatthe laser damaged the fiber in such a way that no light could betransmitted and that the damage seemed to occur instantaneously. This isa typical failure mechanism when the pulse energy is too high.

It is known from prior tests that the SC-5.0-1040 can withstand 1.2 nslaser pulses at 1064 nm having pulse peak power in the range of 10-12kW, and that such pulses generate a broad supercontinuum from 0.5 to 2.4μm in the fiber. This fiber is used for NKT Photonics commercial productSuperK Compact.

The pump laser 20 was applied to an example of the microstructuredoptical fiber of the invention comprising an intermediate tapered fiberlength section in order to prove that this fiber could sustain the pulsepeak power which a normal non-linear fiber cannot.

The microstructured optical fiber used for the experiment had thefollowing characteristics:

d/∧=0.52 in the first fiber length section, the second fiber lengthsection as well as in the intermediate fiber length section;

Characteristic core diameter in first fiber length section: 10 μm;

Characteristic core diameter in second fiber length section: 5 μm;

Length of first fiber length section: app. 1 m (±0.1 m);

Length of taper, viz. intermediate fiber length section: app. 1 m (±0.1m);

Total length of the microstructured optical microstructured fiber: app.15 m (±0.5 m).

Both ends of the microstructured optical fiber were cleaved andcollapsed with a 200 μm±25 μm collapse length, giving end caps.

The input end of the microstructured optical fiber viz. the input end ofthe first fiber length section, was positioned in front of the pumplaser, and a pair of lenses was used to couple the light from the pumplaser into the fiber. The lens pair consisted of a collimating lenshaving a 100 mm focal length and a focusing lens having a focal lengthof 3.5 mm. The two lenses were separated by a distance of app. 30 mm.The lenses were aligned in the x- and y-axis until seed light wascoupled into the core of the microstructured optical fiber. Focus on thefiber was achieved by moving the microstructured optical fiber endtowards the focusing lens until maximum power was measured using athermal detector from Ophir Optronics (Ophir power head model #3A).

The alignment procedure continued until maximum power out of themicrostructured optical fiber was measured. This was achieved around 70mW output power.

Once a supercontinuum was achieved through the tapered microstructuredoptical fiber, the result was recorded using an Optical SpectrumAnalyser (OSA) from ANDO, model #6315B. A spectrum was recorded from 400to 1750 nm with a 5 nm resolution. The spectrum was recorded through anintegrating sphere (Ocean Optics) with a multimode fiber having an Ø1 mmfiber core.

The coupling from the seed laser into the tapered microstructuredoptical fiber operated at maximum achieved output power (app. 70 mW) forapp. 10 minutes in order to verify that the microstructured opticalfiber core would not burn due to the increased pulse peak power. Incontrast when the laser was fed directly into the prior art fiberSC-5.0-1040, the core of the optical fiber was destroyed within a fewseconds once optimal power has been achieved due to the high peak powerand pulse energy of the pump source. A significant portion of themeasured 600 mW input power is believed to arise from non-absorbed 808nm pump light which can be seen in the spectrum. To achieve the bestinjection efficiency into single-mode fiber, the direction, position,size and divergence of the beam from the pump light source areadvantageously all optimized. In the present experiment, is believedthat the lens pair was not optimally aligned with respect to the core ofthe microstructured optical fiber resulting in a significant portion oflight being lost instead of reaching the microstructured optical fiber.Thus, it is believed that considerable more output power is achievable,in the order of magnitude 200-300 mW.

The experiment demonstrates that the first fiber length section havingan enlarged size compared to the second fiber length section andcompared to the standard non-linear fiber is capable of coupling thelight into the second fiber length section which is here a single modepart of the microstructured optical fiber, thus creating asupercontinuum. Thus, light in the first fiber length section and/or inthe intermediate fiber length section does not exit the microstructuredoptical fiber at end of these the respective first and intermediatefiber length sections, but is being coupled into the core of the singlemode structure in the second fiber length section of the microstructuredoptical fiber.

The experiment also demonstrates that the expanded input in the form ofthe first fiber length section having a large first characteristic corediameter is able to sustain a greater pulse peak power than the priorart non-linear supercontinuum fibers. This is seen in that therelatively high peak power provided by the pump laser tends to damagethe input end facet of a prior art standard supercontinuum, non-linearfiber but did not damage the tapered microstructured fiber of anembodiment of the invention.

Thus, the experiment shows that the microstructured optical fiberaccording to the invention is useful for relatively high energy pulsepower compared to prior art fibers used for SC generation.

In an embodiment of the invention, the pump laser 200 is a q-switchedlaser comprising an active feedback to control the emission frequency(repetition rate) of the pump laser, and/or to reduce the timing jitterbetween the pump laser pulses.

Passive Q-switched pump lasers comprising an absorbable absorber (e.g. aCr:YAG Crystal) and a laser diode to pump the absorbable saturator (e.g.a CW laser diode with a peak wavelength at 808 nm) are well known in theart. Here pulses are emitted from the pump laser whenever the absorbablesaturator becomes transparent (bleaches) thus changing the transmissionthrough the crystal from a low transmission value to a high transmissionvalue. This is e.g. described in the W. Koechner's book “Solid-StateLaser Engineering” on pp. 522-523 (Springer, Sixth revised and UpdatedEdition, 2010, ISBN-13: 978-14419-2117-8). The repetition rate of theemitted pulse train is random in nature due to a number of limitationsin the seed laser cavity (noise, fluctuation in input power, temperaturefluctuation, etc.).

In an aspect the inventors has found that pump light source comprising asystem for active feedback as shown in FIG. 9 in principle can beapplied as pump source for any kind of supercontinuum fiber, such asnon-tapered microstructured supercontinuum, in a supercontinuum lightsource. In an embodiment the pump light source comprises a q-switchedlaser comprising an active feedback system for active feedback asdescribed below.

The pump light source generally provides a very stable emissionfrequency (repetition rate) of the pump laser and/or a reduced timingjitter between the pump laser pulses.

In an embodiment the control is obtained by modulating the output fromthe laser diode. This might e.g. be obtained by detecting whenever apulse is emitted from the pump laser 200. The feedback signal is used toswitch the laser diode from “ON” (emitting light) to “OFF” (noemission).

FIG. 9 is a diagram of an embodiment of the pump light source comprisinga system for active feedback. As mentioned this pump light source mayadvantageously be used as a part of the supercontinuum light source ofthe invention but it may in principle be applied as a light source inany optical systems.

In the embodiment illustrated in FIG. 9, the pulse detection is obtainedby placing a photodiode so that it receives part of the emitted pulsesfrom the pump light source. The photodiode should be sensitive to lightemitted from the pump laser (1064 nm), but not sensitive to lightemitted from the laser diode (808 nm) as light from the laser diode maygive rise to noise in the detection. This is obtained by either carefulselection of photodiode type or by placing an appropriate filter infront of the detector.

When the laser diode 30 is turned “ON” it will start to pump energy intothe pump laser cavity 31. No light is emitted from the pump laser untilthe saturation state of the absorbable saturator is changed allowinglight from the seed laser cavity to emit a pulse. When the pulse isemitted from the pump laser it will be detected by the photodiode 32.The signal from the photodiode is sent to a control unit 33, which sendsa signal to the laser diode to shut it “OFF” thus preventing furtherlight to be pumped into the laser cavity.

With the laser diode turned “OFF” no light is pumped into the pump lasercavity, and consequently no further seed laser pulses are emitted. Thelaser diode may be turned to “ON” again whenever a new pulse isrequested by the supercontinuum laser system. This request may either beat a constant frequency or by a user trigger input. The maximumobtainable repetition rate is achieved when the requested frequencyexceeds the pump capacity of the pump source laser. This happens when anew pulse is requested before the previous pulse is yet emitted from thepump laser. The laser diode will then operate continuously providing themaximal repetition rate possible by the pump laser.

During the laser diode “OFF” state the laser may be kept on a thresholdlevel where no light is emitted from the laser. Keeping the laser diodeat threshold will enable a faster rise time of the laser diode unit andthus a faster response of the pump laser.

The use of active feedback enables full control of the pump laserrepetition rate up to the level where the laser diodes operatescontinuously, which typically occurs at a pump laser repetition rate of40-50 kHz. Furthermore a much more stable pulse-to-pulse signal will beobtained as the pulse-to-pulse jitter is only dependent on the amount ofenergy (and thus time) the laser diode requires in order to providesufficient energy for the pump laser to emit a pulse. The timing jitterbetween pulse emissions from the pump laser is reduced to a few hundredns (100-300 ns) where a normal passive q-switched pump laser may have apulse-to-pulse jitter in the range of a few ms (2-10 ms). As the pumplaser only acquires the amount of energy sufficient in order to emit asingle pulse at a time, the emitted pulse itself is expected to be lesssensitive to pulse jitter such as pulse width jitter and pulse amplitudejitter.

In one embodiment of the invention the pump laser has a variablerepletion rate.

In one embodiment of the invention the pump laser pulses are externallytriggered. This could e.g. by used in connection with a measurementmethod having a finite sampling time, where it is advantageous havingthe same number of pulses within each of the sampling intervals. Oneexample is where for hyperspectral imaging with a given shutter time.

In one embodiment of the invention the pump laser timing jitter is suchas less than about 1 ms, such as less than about 500 ns, such as lessthan about 300 ns, such as less than about 200 ns.

The supercontinuum light source of the invention has been found to behighly suitably for performing photoacoustic imaging due to its abilityto generate supercontinuum pulses with very high pulse energy.

The invention also comprises a photoacoustic imaging system comprising asupercontinuum light source as described above, a detector for detectingultrasonic emission waves and an image processor for forming an image ofthe detected ultrasonic waves. The detector can be any that is suitablefor ultrasonic emission waves, preferably with a high sensitivity e.g.such as ultrasonic emission waves used in prior art photoacousticimaging systems. The processor for forming an image of the detectedultrasonic waves can for example be in prior art photoacoustic imagingsystem.

Photoacoustic imaging is well known and is based on the photoacousticeffect. In photoacoustic imaging, non-ionizing laser pulses aredelivered into biological tissues. Some of the delivered energy will beabsorbed and converted into heat, leading to transient thermoelasticexpansion and thus wideband (e.g. MHz) ultrasonic emission. Thegenerated ultrasonic waves are then detected by ultrasonic transducersto form images. It is known that optical absorption is closelyassociated with physiological properties, such as hemoglobinconcentration and oxygen saturation. As a result, the magnitude of theultrasonic emission (i.e. photoacoustic signal), which is proportionalto the local energy deposition, reveals physiologically specific opticalabsorption contrast. 2D or 3D images of the targeted areas can then beformed.

The invention further comprises a method of performing photoacousticimaging of a biological tissue the method comprising providing aphotoacoustic imaging system as described above and deliveringsupercontinuum laser pulses from said supercontinuum light source tosaid biological tissue and collecting ultrasonic emission waves fromsaid biological tissue by said detector and forming the image using saidimage processor.

Due to the very high power and pulse energy of the supercontinuum lightsource of the invention, the supercontinuum light source has furtherbeen found to be very suitable for use in multimodal imaging, e.g.combining photoacoustic and optical coherence tomography (OCT) imaging.The invention therefore also relates to a multimodal photoacoustic andoptical coherence tomography (OCT) image acquisition system comprisingphotoacoustic imaging system as described above combined with an OTCimaging system, wherein the photoacoustic imaging system and the OTCimaging system using said supercontinuum light source as a common lightsource, said multimodal photoacoustic and optical coherence tomography(OCT) image acquisition system preferably further comprises a detectorfor collecting reflected light and an image processor for forming animage of the detected reflected light.

Optical coherence tomography is an established medical imagingtechnique. It is widely used, for example, to obtain high-resolutionimages of the anterior segment of the eye and the retina. OTC isadvantageous for delivering high resolution because it is based onlight, rather than sound or radio frequency. An optical beam is directedat the tissue to be analyzed, and a small portion of this light thatreflects from sub-surface features is collected. Most of the light isnot reflected but, rather, scatters off at large angles. The OCT uses atechnique called interferometry to record the optical path length ofreceived photons allowing rejection of most photons that scattermultiple times before detection. Thus OCT can build up clear 3D imagesof thick samples by rejecting background signal while collecting lightdirectly reflected from surfaces of interest. The invention is inparticular suited for spectral domain OCT.

The multimodal photoacoustic and optical coherence tomography (OCT)image acquisition system preferably comprises a filter for selecting aspectral portion of the light beams to be applied in the OTC. Due to thevery high power the multimodal photoacoustic and optical coherencetomography (OCT) image acquisition system advantageously comprises anintensity filter for reducing the intensity applied in OCT in dependenceon the subject to be analyzed. Where OCT is applied for analysis ofsensitive tissue, such as eye tissue, it is desired that the intensityis kept relatively low in order not to damage the tissue.

It should be emphasized that the term “comprises/comprising” when usedherein is to be interpreted as an open term, i.e. it should be taken tospecify the presence of specifically stated feature(s), such aselement(s), unit(s), integer(s), step(s) component(s) and combination(s)thereof, but does not preclude the presence or addition of one or moreother stated features.

All features of the inventions including ranges and preferred ranges canbe combined in various ways within the scope of the invention, unlessthere are specific reasons for not to combine such features.

Some embodiments have been shown in the foregoing, but it should bestressed that the invention is not limited to these, but may be embodiedin other ways within the subject-matter defined in the following claims.Various modifications and applications may occur to those skilled in theart without departing from the true spirit and scope of the invention asdefined in the appended claims.

The invention claimed is:
 1. A supercontinuum light source comprising: amicrostructured optical fiber for generating supercontinuum lightresponsive to being pumped, and a pump light source arranged to pumpsaid microstructured optical fiber with pump light having a firstwavelength, λ₁; wherein: said microstructured optical fiber has a lengthand a longitudinal axis along its length and comprises a core regionthat is capable of guiding light at the first wavelength, λ₁, along thelongitudinal axis of said microstructured optical fiber and a firstcladding region surrounding said core region; said microstructuredoptical fiber comprises a first fiber length section, a second fiberlength section as well as an intermediate fiber length section betweenthe first and second fiber length sections; the core region of saidfirst fiber length section has a first characteristic core diameter W₁,the core region of said second fiber length section has a secondcharacteristic core diameter W₂, where said second characteristic corediameter W₂ is smaller than said first characteristic core diameter W₁,and where said second characteristic core diameter W₂ is substantiallyconstant along said second fiber length section; the characteristic corediameter of the core region of said intermediate fiber length section ofthe microstructured optical fiber is tapered from the firstcharacteristic core diameter W₁ to the second characteristic corediameter W₂ over a tapered length L_(i); and wherein said second fiberlength section has zero dispersion at a wavelength ZDW₂, where ZDW₂ isup to about λ₁+50 nm, and wherein said second fiber length section hasanomalous dispersion at wavelengths above said ZDW₂.
 2. Amicrostructured optical fiber for generating supercontinuum light uponfeeding of light having a first wavelength λ₁, the microstructuredoptical fiber having a length and a longitudinal axis along its lengthand comprises a core region that is capable of guiding light at saidfirst wavelength λ₁ along the longitudinal axis of said microstructuredoptical fiber and a first cladding region surrounding said core region;said microstructured optical fiber comprises a first fiber lengthsection, a second fiber length section as well as an intermediate fiberlength section between the first and second fiber length sections; thecore region of said first fiber length section has a firstcharacteristic core diameter W₁, the core region of said second fiberlength section has a second characteristic core diameter W₂, where saidsecond characteristic core diameter W₂ is smaller than said firstcharacteristic core diameter W₁, and where said second characteristiccore diameter W₂ is substantially constant along said second fiberlength section; the characteristic core diameter of the core region ofsaid intermediate fiber length section of the microstructured opticalfiber is tapered from the first characteristic core diameter W₁ to thesecond characteristic core diameter W₂ over a tapered length L_(i); andwherein said second fiber length section has zero dispersion at awavelength ZDW₂, where ZDW₂ is up to about λ₁+50 nm, and wherein saidsecond fiber length section has anomalous dispersion at wavelengthsabove said ZDW₂.
 3. The microstructured optical fiber according to claim2, wherein said first characteristic core diameter is larger than about7 μm.
 4. The microstructured optical fiber according to claim 2, whereinZDW₂ is in the range from about λ₁−150 nm to about λ₁+50 nm.
 5. Themicrostructured optical fiber according to claim 2, wherein said firstwavelength λ₁ is up to about 1100 nm.
 6. The microstructured opticalfiber according to claim 2, wherein the first fiber length section haszero dispersion at a wavelength ZDW₁ where ZDW₁> about λ₁+20 nm.
 7. Themicrostructured optical fiber according to claim 2, wherein said firstcharacteristic core diameter is larger than about 1.5 times the secondcharacteristic core diameter.
 8. The microstructured optical fiberaccording to claim 2, wherein said first characteristic core diameter islarger than about 10 μm.
 9. The microstructured optical fiber accordingto claim 2, wherein said second characteristic core diameter is largerthan about 3.5 μm.
 10. The microstructured optical fiber according toclaim 2, wherein said first cladding region comprises a microstructure,said microstructure comprising a plurality of microstructure elementshaving a microstructure element diameter, d_(f), said microstructureelements being arranged at a pitch ∧.
 11. The microstructured opticalfiber according to claim 10, wherein the relative size (d_(f)/∧) of themicrostructure elements is larger in the second fiber length section ofthe microstructured optical fiber than in the first fiber length sectionof the microstructured optical fiber, said relative size being the ratiobetween the diameter (d_(f)) of the microstructure elements and thepitch ∧.
 12. The microstructured optical fiber according to claim 10,wherein the relative size (d_(f)/∧) of the microstructure elements ischosen so that the first fiber length section is a single mode fiber atleast at the first wavelength λ₁.
 13. The microstructured optical fiberaccording to claim 10, where said microstructure elements are arrangedin a plurality of layers within the first cladding region.
 14. Themicrostructured optical fiber according to claim 10, where saidmicrostructure is at least partially maintained along the first andsecond fiber length sections of the microstructured optical fiber. 15.The microstructured optical fiber according to claim 2, where said coreregion comprises a material doped with at least one rare earth element.16. The microstructured optical fiber according to claim 2, wherein saidcore region has an effective refractive index n_(core), and, ii) saidfirst cladding being arranged for guiding light at a wavelength λ₃, saidfirst cladding having an effective refractive index n_(first-clad), andiii) said second cladding having an effective refractive indexn_(second-clad) wherein n_(core)>n_(first-clad)>n_(second-clad) andλ₁>λ₃.